Thank you for reaching out to us.
We appreciate the sensitive nature of this question.
If a woman finds blood during relations or immediately afterwards, there is a halachic issue. In contrast, there is no specific halachic concern about a couple having relations prior to the onset of the woman’s period, as long as their timing is not on an onat veset.
In general, when a woman has staining that does not make her niddah, we often recommend abstaining for a day or so as a precaution against a flow beginning during relations. This is not a strict halachic requirement, though, and light brown is not a niddah color, so we don’t usually make this recommendation to women with staining of this shade. If, however, a woman consistently has this staining within a set interval of the onset of her flow, then she might set a veset haguf. (Please get back to us if you think this is relevant here. It didn’t sound like it, from what you wrote.)
We hope that this is clear and puts you more at ease. Please be in touch with any further questions.